by
Kaufmann, Jerome; Schwitters, Karen;

Answer

$61+0i$

Work Step by Step

First of all, recall that $i=\sqrt{-1}$, so $i^{2}=-1$
Also, since $(a+b)(a-b)=a^{2}-b^{2}$, we obtain:
$(5-6i)(5+6i)$
=$(5)^{2}-(6i)^{2}$
=$25-36i^{2}$
=$25-36(-1)$
=$25+36$
=$61$
Since we have to write the answer in the form $(a+bi)$, we write:
$61=61+0i$